1. Field of the Invention
The present invention relates to an improvement in a polyamide base binder which is used in the metal powder injection molding process.
2. Description of the Related Art
In the metal powder injection molding process, a metal powder material is mixed with a binder, followed by molding, degreasing and sintering, to prepare a molded article. This process has advantages over the other general powder metallurgical processes in that an article having a complicated shape can be molded by a single step and the subsequent machining or after-treating steps can be eliminated. Thus, the process is particularly suited for the production of relatively small size metal parts, and has a considerable merit in its low production cost.
However, since a relatively large quantity of binder was used in such a metal powder injection molding process, collapsing, cracking or carbonization of the used binder could occur in the degreasing and/or sintering steps, which posed problems to be solved.
It is also known to prepare, for instance, alumina ceramics by the utilization of similar technology. When the metal powder injection molding process is compared with the similar technology for the production of alumina ceramics, the true specific gravity of the metal ranges generally from 7 to 8 (such as for iron base alloys), which is about 2 times as high as that of the alumina ceramics. In view of the difference in specific gravity, the molded article produced by the metal powder injection molding process tends to be deformed (sometimes by collapsing) due to its own weight during the degreasing steps. When a large quantity of a high polymer material is used in the binder to prevent deformation, carbonized portions are formed during the degreasing and/or sintering steps which result in a product of inferior quality.
Thus, the result of the metal powder injection molding depends on the selection of used binder and it is an important factor to use a superior binder to obtain molded products of high quality.
In the conventional technology, generally used binders are composed of plural binder components having different melting points. The binder components of each known binder has different melting points to ensure gradual and continuous flow-out of the liquefied binder composition during the degreasing step, the binder composition flowing out of the molded article being absorbed by an absorbent. A typical example of such binder composition is disclosed in Japanese Patent Publication No. 48563/1986 . However, when such a binder is combined with a metal powder having a high specific gravity, the molded body tends to deform due to the weight of the metal powder itself during the degreasing step as the binder components are liquefied.